Periodic signals are common in nature and machines. Quasi-periodic signals are signals that are significantly repetitive without being exactly periodic. Hereinafter, substantially periodic signals include periodic and quasi-periodic signals. A number of biological activities such as heart-beats and respiration, industrial automation processes, and consumer products such as hand-mixers and cooling fans generate substantially periodic signals.
High Speed Imaging Hardware
One of the constraints for recording a high speed video signal is a short exposure time to reduce motion blur. This requires increased illumination or the use of high gain imagers. However, in many applications bright light cannot be used. Furthermore, high speed cameras are expensive.
Strobing
Conventional stroboscopes synchronize a periodic short burst of illumination to the periodic motion in the scenes so that the motion appears stationary, or almost stationary. If the synchronization is exact, the motion is stationary. When the synchronization is slightly slower or faster than the motion, the motion appears to go slowly backwards or slowly forwards. To be able to achieve these effects, the period of the motion must be known in advance. Also, because the illumination is a periodic short burst, the total illumination time is very small requiring extremely bright sources. Therefore, most stroboscopes use very bright flashes to compensate for the reduced illumination.
Processing
Many computer vision applications are dealing with substantially periodic motion. For example, a period trace provides a description of temporal variations in periodic motion, and can be used to detect motion trends and irregularities.
Structured-light can be used to obtain dense depth and color samples for moving and deformable surfaces undergoing repetitive motions. Another alternative method to obtain high temporal and spatial resolution of a signal is via a hybrid imaging device, which includes a high spatial resolution digital camera in conjunction with a high frame-rate, but low resolution video camera. Also, coded exposures can be used to reduce blurs in images caused by linear motion.
It is desired to increase temporal resolution of substantially periodic signals acquired by cameras without the use of bright illumination or the use of expensive high-gain sensors.